# Vibration Test Machine–Purpose and Basic Knowledge

Mechanical vibration means reciprocating movement of an object near its position. During this process, some physical parameters of the object like replacement, speed etc will change which cause noise pollution, increase the fatigue and wear of the components, shorten the service life of the machine and brings harm to daily life and engineering projects. Now we are going to show you some common sense of the vibration test that you much know and some preparation before test.

1.What is vibration?

Vibration is the oscillating phenomenon of motion quantity (displacement, speed and acceleration) in mechanical system.

2.Purpose of vibration test

The purpose is to simulate a series of vibration phenomena and test whether the product can stand the vibration environment in transportation and usage. The essence is to confirm the reliability of the product and screen out defective one, so as to improve the product quality.

3.Vibration classification

Vibration divides into two kinds: specific vibration and random vibration.

4.Sine vibration

A periodic motion that can be expressed by Sinusoidal function.

Such us the vibration caused by rotation, pulsation, oscillation(in ship, aircraft, vehicles, space vehicles) is sinusoidal.

5.Purpose of sine vibration

The purpose is to simulate vibration and influence of electrical and electronic products in transportation, storage and usage so as to examine their adaptability.

6.Test conditions of sine vibration

The test conditions are determined by frequency, vibration quantity and test duration (test times).

7.Vibration frequency range

The range represents the vibration test from a specific frequency to another.

For example: The frequency range is 5~50Hz, indicating the reciprocating sweep frequency.

8.Frequency

Vibration times for one second. Unit: Hz

9.Vibration measurement

Vibration measurement: expressed by acceleration, velocity and displacement.

Acceleration: A vector representing the reciprocal of velocity to time. Unit: g or m/s2.

Speed: Numerically equal to the distance passed in a unit time.

Displacement: A vector representing the position of an object relative to a reference system. Unit: mm

10.Duration

Vibration test time;

The number of times represents the number of sweep cycles required for the whole experiment.

11.Sweep frequency cycle

A round-trip vibration within a specified frequency range.

For example, 50Hz→5Hz→50Hz→5Hz, scan from 5Hz to 50Hz and then 5 Hz.

12. Gravitational acceleration

The acceleration of an object on the earth's surface due to gravity.

1gn=10m/s2 (GB/T 2422-1995 term for Environmental Testing of Electrical and Electronic Products)

13. Scanning mode

Liner scanning: How many Hz sweep in unit time; this is used pinpoint the resonance frequency. Unit: Hz/s or Hz/min

Logarithmic scan: frequency changes to logarithmic change, scanning rate: oct/min

14. Sweep speed

Speed from lower frequency to higher frequency. There are following conditions:

(1)Oct/min: octaves per minute

Eg: 1 oct/min: it takes ONE minute from 5Hz to 10 Hz; It takes ONE minute from 10Hz to 20Hz

(2)Min/sweep: sweep frequency per minute.

Eg: 5~500Hz: sweep speed ONE minute/sweep, it takes ONE minute from 5Hz to 500Hz

(3)Hz/s: Hertz per second

Eg: 5~10Hz sweep speed: 1Hz/s, it takes ONE second from 5Hz to 6Hz; it takes ONE second from 6Hz to 7Hz.

15.Vibration direction

Unless special requirement, vibration tests shall be carried out in three vertical directions of the product.

The long edge of the product is defined as X axis, the short edge is Y axis, and the normal arrangement of the product is Z axis.

16. Crossover frequency

The frequency from one vibration characteristic to another in the test.

Eg. Crossover frequency changes from constant displacement→frequency relation to the constant acceleration→frequency relation to the constant acceleration.

17. Why resonance search?

In general, there are various components on the object to be tested, and each of the different components has its own resonant frequency, at the same time it produces different resonance frequencies and magnification rates when the vibration occurs because of the different shapes, weights, and fixed modes. Therefore, it is necessary to search the resonance characteristics of a specific component, and then to perform the resonance test of the resonance point according to its characteristics.

18. Resonance frequency

The frequency at which the product resonates.

19. Fixed frequency

Test of various vibration parameters on a required and fixed frequency.

20. Usage of fixed frequency

(1)Resonance frequency vibration: the vibration that add amplitude after checking the obvious resonance frequency point, so as to test durability of product under such condition.

(2)Predetermined frequency resistance vibration

This test adopts under a preset frequency of using condition of test product. It is to examine the tolerance of product at a predetermined risk frequency.

21.Critical frequency:

the frequency at which mechanical resonance and other response effects such as tremor occurs due to vibration resulting in abnormal or deteriorating performance of the product.

22. Random vibration

For any given moment in the future, its instantaneous value cannot be predetermined. In a normal transportation environment, the vibration generated by a vehicle in transit is random. Even if at the same time, each frequency has its own vibration measurement. When performing the random wave vibration test, there is vibration value at the same time and different frequencies.

23.Acceleration spectral density

It represents how the acceleration square value of each frequency distributes. It's usually expressed as ASD, unit: m2/s3 or (m/s2)/Hz

24.Power spectral density

It represents how the power of each frequency component of a random signal is distributed in frequency domain. It's usually expressed in PSD, unit: G 2/Hz.

Conversion of acceleration spectral density to power spectral density:

1 G2/Hz=(9.8 m/S2)2=96.04 m2/S3

25. Standards of sinusoidal vibration

GB/T 2423.10-2008 Environmental Tests for Electrical and Electronic Products Part 2: Test Methods Fc: Vibration (Sinusoidal);

IEC 60068-2-6-2007 Basic Environmental Test Code. Part Second: Test. Section Sixth: Test Fc: Vibration (Sine Wave);

ISO 83 18: 2000 Package. Full Packing of Transport Packages and Unit Goods. Sinusoidal Vibration Test with Variable Frequency;

GB/T 4857.10-2005 Basic Tests for Packaging and Transport Packages-Part 10: Test Methods for Sinusoidal Frequency Conversion Vibration;

26.Standards of random vibration

GB/T 2423.56-2006 Environmental Testing of Electrical And Electronic;

Products-Part 2: Test Methods, Fh: Wideband Random Vibration (Digital Control) and Guidelines;

IEC 60068-2-64-2008 Basic Environmental Test Code. Part 2-64: Testing. Test Fh: Vibration, Wideband Random Sampling;

Standard Test Method for Random Vibration Detection of ASTM D4728-2006 Marine Container;

GB/T 4857.23-2003 Test Method for Random Vibration of Packaging and Transport Packages;

27.Root-mean-square

The value in f1 and f2 inerval, expressed as‘RMS’

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